I’m having the same exact issue as OP.
I got to about 90% download, and my internet went out (my school uses Cisco and the client sucks so I do 90min access at a time).
Signed in again for another 90 minutes and the stupid gw2 client keeps giving me the “download failed!…(2)” crap.
I’ve tried Windows run, disabling anti-virus, turning off windows defender…everything I can think of…
Gonna watch the thread, I’ll update if I figure this out. Good luck! lol